Engineers have successfully reverse-engineered MLow, a proprietary low-bitrate audio codec developed by Meta for WhatsApp, Messenger, and Instagram. The project aimed to create a native Go implementation without CGO or external dependencies, ensuring compatibility with Meta's ecosystem.
MLow is a hybrid codec based on the Opus framework, utilizing both CELT-based transform modes for general audio and CELP-based speech modes. The researchers discovered that while static analysis suggested a simple SILK derivative, the codec's actual behavior required a more complex approach to handle varying network conditions and packet loss.
The team employed an 'oracle' method, comparing intermediate signal processing states between the original library and their implementation. This process allowed them to identify exact points of divergence, leading to a decoder that matches the original bit-for-bit in transform modes and handles speech modes with high accuracy.
Performance optimizations included precomputing FFT tables and reducing memory allocations, resulting in a codec that runs approximately 14 times faster than real-time for encoding and 270 times faster for decoding. The final implementation is written in pure Go and avoids SIMD to maintain bit-perfect compatibility with the original Meta codec.
